Another reason to not leave vintage tube amps unattended.....

[AdamAnt316]

I had the amp on for a few hours after my post, and it didn't red-plate again, and sounded fine. I'm guessing it was a dirty/loose contact in the socket for the tube which red-plated. I'm planning to apply DeOxit to the sockets, and burnish/retension the contacts. My ST-70 has a proper fuse installed, and I'm guessing it would've popped if I'd been away from the amp, but I hate having to replace parts without a good reason, and a EL34 melted by a dirty/loose socket contact qualifies as such in my book.

As for running equipment 24/7, I generally only run my equipment when I'm sitting in front of it. About the only exception is my Superphon Revelation preamp, which is meant to be powered on all the time (doesn't have a power switch, just one labeled MUTE/PLAY). Occasionally I'll leave something turned on for a little while when I'm not nearby, but I do my best to avoid that. My ST-70 and turntable are connected to an old computer power strip which has individual buttons for each outlet, as well as a main power button for the entire strip. I turn off the main switch each night after I'm done on my computer.
-Adam

 

[Alan0354]

Ono more thing, tap the problem tube with a wood stick to make sure there's nothing loose inside. Jiggle the tube when it is on while monitoring the cathode current. Rule out tube problem.

 

[Dennis Gardner]

Tube amps, TVs, radios, etc. were rarely the source of house fires for over half a century of being the primary electronic devices in our lives. More often than not extension cords or inwall wiring were the danger. We sure are becoming a bunch of worry warts........how did they ever live with these "tube" devices all over their house? They must have not gotten any sleep for years.......

 

[bberkom]

My Fisher 500c did the same thing once, randomly redplating one tube. I checked all voltages and solder joints associated with the socket, and I could never get it to recur. I cleaned and tensioned the sockets, and it's been fine. Not good for the tube of course, but no real harm done.

 

[radioactive]

Hello everyone. Earlier tonight, after I took my computer out of sleep mode, I turned on my Dynaco ST-70 as I tend to do almost every night. Soon after, I then proceeded to make a phone call, and while I was on the phone, I started hearing some odd crinkling noises. I took a look at my ST-70, and one of the EH EL34s in the right channel had a nice big red spot on the plate!

I quickly hit the power switch, and watched the red spot cool off. I estimate that it'd been like that for maybe 30 seconds. Once the tubes cooled down enough, I pulled both tubes for the right channel out of their sockets, swapped them around, and powered the amp back up. The red spot has yet to come back, so I'm guessing it was due to a dirty contact in that particular socket, which swapping the tubes has cured (for now).

Fortunately, the incident doesn't seem to have affected the sound quality, but I can only imagine what would've happened if I hadn't been sitting in front of the amp at the time. This is the first time in the 15+ years I've had this amp that something like this has occurred. Anyway, just another reminder to not leave these older tube amps unattended if you can help it, lest something like this happen..........
-Adam

hi adam,
sorry to hear about the dynaco but id still trust the collins gear left on 24/7

 

[airtime]

My NAD C275BEE SS amp generates as much heat as my VTA ST-70.

If I'm not using it it's not on. I'm not that committed to audio to risk burning down my house.

 

[gadget73]

I usually shut stuff off when not using it, but I've forgotten. I had a stack of records finish last nighht and I realized the Sherwood was on, patiently idling away probably 3 hours later. Its fully rebuilt so I don't worry about it so much.

 

[Legrace]

After 2 red plate events I no longer leave my monoblocks unattended. They are only slightly over a year old so not just a vintage gear issue. My kingdom for a decent quad of power tubes. Suggestions for hardy 6550's, KT88, or KT120?

 

[crispycircuit]

Each tube needs it's own bias adjustment. Dyna's cathode sharing bias resistor is a bad idea for modern tubes. It doesn't take much for 1 tube to hog all the current and go into run away. Perfect matched pairs are only perfect for a short time. The more you use them, the more they drift apart. Seperate bias adjustments keep ageing tubes pulling the same current.

 

[maxhifi]

Don't lose sleep over this, just make sure the mains fuse is the right value. This little event is pretty normal. Chinese tubes from the 90s were the worst for this kind of failure! What we have now is way better.

Good idea to clean and retension the octal sockets, especially pin 5 (G1)

The way I look at it from a fire safety perspective, is imagine the amp was a big power resistor, just barely smaller than would blow the main fuse. Now. Would this resistor catch the surroundings on fire? If yes, change the surroundings until it is no longer the case.

 

[MurrayLives]

Each tube needs it's own bias adjustment. Dyna's cathode sharing bias resistor is a bad idea for modern tubes. It doesn't take much for 1 tube to hog all the current and go into run away. Perfect matched pairs are only perfect for a short time. The more you use them, the more they drift apart. Seperate bias adjustments keep ageing tubes pulling the same current.

I definitely prefer manually doing it. Biasing my Manley’s 20 tubes is a PITA, but not enough for me to wish for any other scheme.

Definitely no such thing as a matched pair/quad/whatever.

 

[jharris]

It's not that bad, so the tube burn, so what? I still say, if the transformer burn, it got to be defect and it's a ticking time bomb anyway.

Depends on how fuses are set up and how stiff the power supply is. Even the best, like-new-condition OPT is not meant to handle dozens of amps through it. The main fuse should go first, but depending on capacitor size and stuff like that, there's a lot of power available no matter how fast it blows.

Cathodes and/or OPT wires should probably be fused themselves.

I'd say if a failure burns a transformer, something about the amp was a ticking time bomb and should have been designed differently, but maybe not the transformer itself.

 

[Patrice B]

Just to add to the OP story:

A couple of years ago, my Fisher was left on when we were out for around three weeks, no damage... Phew!

Also left a VOM amp unrestored playing music while being on phone: a big smoke show and an exploded can cap puking liquid as a result...

 

[Alan0354]

Depends on how fuses are set up and how stiff the power supply is. Even the best, like-new-condition OPT is not meant to handle dozens of amps through it. The main fuse should go first, but depending on capacitor size and stuff like that, there's a lot of power available no matter how fast it blows.

Cathodes and/or OPT wires should probably be fused themselves.

I'd say if a failure burns a transformer, something about the amp was a ticking time bomb and should have been designed differently, but maybe not the transformer itself.

The more fuse the better. But if there is no defect in the transformer, it's not easy to burn. Those wires in the primary is not exactly small, they looks bigger than a 5A fuse wire. The static resistance of the primary is about 100ohm or over, even at 500V, the max you can get out of shorting is 5A, it's not something that will light up the wires. The main fuse better blow when drawing 5A X 500V = 2500W of power!!!

Same idea goes with the power transformer, those wires are not exactly hair thin. If there is no defect in the transformer, it's hard to imagine it will burned by a short. If there is defect like a cold solder between the copper wire and the final wire that comes out of the transformer, then all bets are off. Other than that, the transformer has to pass large current for a long time to heat up and burn the insulation before it goes bad. It's a long period of time thing, not a split second thing.

 

[s-petersen]

The main reason the Power transformers burn out is the output tubes, or capacitors draw too much current for too long without shorting, exceeding the transformers rating and overheating it, then the windings start to short, finally blowing the fuse when it is already too late.

 

[solderjunkie]

Tube amps, TVs, radios, etc. were rarely the source of house fires for over half a century of being the primary electronic devices in our lives. More often than not extension cords or inwall wiring were the danger. We sure are becoming a bunch of worry warts........how did they ever live with these "tube" devices all over their house? They must have not gotten any sleep for years.......

Just to add to the OP story:

A couple of years ago, my Fisher was left on when we were out for around three weeks, no damage... Phew!

Also left a VOM amp unrestored playing music while being on phone: a big smoke show and an exploded can cap puking liquid as a result...

I had a similar experience when i discovered my teenage son left a Curtis Mathes console power amp I converted into an integrated stand-alone powered on for two weeks. And those amps naturally run hot. Luckily no harm to our house or the amp, but I was livid.

 

[jharris]

Those wires in the primary is not exactly small, they looks bigger than a 5A fuse wire. The static resistance of the primary is about 100ohm or over, even at 500V, the max you can get out of shorting is 5A, it's not something that will light up the wires. The main fuse better blow when drawing 5A X 500V = 2500W of power!!!

Same idea goes with the power transformer, those wires are not exactly hair thin. If there is no defect in the transformer, it's hard to imagine it will burned by a short.

I've put a lot of current through a cheap Champ-style guitar OPT due to a bad tube... combined with my failure to protect against the bad tube. It went on for long enough for the OPT to be HOT to the touch, yet I didn't kill it. So you're right, in some circumstances. But it's worth noting the fault current was limited by the OPT and my power supply to the extent it didn't blow my main fuse, and if this had gone unchecked for longer the OPT (or PT) would eventually be damaged.

After all, it doesn't need to get hot enough for the wire to melt like a fuse. Just hot enough to compromise the insulation of the magnet wire, causing an internal short. As you note, internal solder joints may be a weak point if they're not high quality, but even if they are you'll probably get them melting long before the actual wire acts like a fuse.

 

[maxhifi]

I know some people frown on this practice, and for good enough reason, but my own technique is to use 1/4W 10 ohm metal film resistors from cathode to ground for the output tubes.

They serve the dual purpose of measuring cathode current accurately, and metal film resistors fail open, more or less like fuses, and are really cheap.

Carbon resistors can short, but metal film just vaporize open. This assumes that your power supply had enough juice to open the resistors if a tube shorts, but the same issue exists with fuses.

The best insurance though is just use good quality tubes, periodically check the condition of your amp, and don't leave it unattended for too long.

 

[Alan0354]

Guys,
If this is such concern, it's time to modernize to protect your valuable tube amps. This is some simple example of how to protect the transform from a leaky cap or tube:



The left one is to protect PT. you put a low value resistor of your choice. You use a comparator that has a preset reference so what the voltage developed due to leaky cap exceed the REF voltage, you trigger the comparator and turn off the relay to stop the power.

The right one is protecting the output transformer, idea is just the same.


Of cause, the real circuit is not that simple, you need to have a time delay before turning on or else it will keep clicking as the relay is off, the voltage across the resistor goes below the REF, the relay turns on and trip the comparator again and again. BUT, the idea is to monitor and trip the power if there is a fault condition.

Most SS amp has this kind of protection, not for the transistor as much as to protect the speakers.

REMEMBER, this is only an idea, NOT an actual working circuit. It's more complicated than that, I am not going to spend the time to design a real working circuit.

 

[gadget73]

The problem is you'd have to account for max current usage, so basically both channels driven to full power. Unless the leaky cap exceeds that, its not going to trip. I expect one tube with a slightly faulty cap may not be enough to do it.

The cathode resistor thing can definitely help, the trouble is that resistors are not the most predictable fuses. The 1/4 watt precision ones seem to vaporize easier than carbon film do though. Vastly better than nothing though.